More particularly the invention is concerned with a locked oscillator type of detector in which the detector loop is made a frequency determining element of a first oscillator and which comprises a second internal oscillator which is coupled to the first and its frequency adjusted such that by virtue of the mutual coupling the oscillator frequencies lock together. A detector of this type is disclosed in British Patent Specification No. 1,338,062 of Wilcox. When a vehicle influences the detector loop oscillator the frequencies of the two oscillators remain locked but there is a shift of their relative phase which is detected and processed to provide an indication of the presence of a vehicle.
In known locked oscillator detectors, the detector loop is made an element of a conventional tuned circuit for the first or loop oscillator and the internal oscillator has to be adjusted in frequency until the oscillators lock at or near the natural frequency of the loop oscillator. The natural frequency can vary widely depending on the loop requirements of each installation and the internal oscillator has to be manually tuned in each case.